Exposure to environmental synthetic estrogens, such as bisphenol A (BPA), has been implicated to contribute to the increasing incidence of breast cancer. Bisphenol A is a most pervasive chemical in modern life as a component of polycarbonate plastics and epoxy resins used widely for food and beverage containers and dental sealants. Perinatal exposure to low, environmentally relevant doses of BPA in rodents resulted in induction of preneoplastic ductal hyperplasias, carcinoma in situ, and increased susceptibility to tumorigenesis. However, the underlying mechanism for these observations is unclear. The murine mammary stem cells (MaSCs) have the potential to drive mammary gland development during puberty, and growth and remodeling during pregnancy. Recent lineage tracing studies also indicated the presence of a hierarchy of stem cells in the murine mammary gland. Significantly, these distinct unipotent basal and luminal MaSCs have been matched with different subtypes of breast cancer by their specific gene-expression signatures. Furthermore, modulation of oncogenes and tumor suppressors has been shown to increase stem cell compartment and self-renewal function of MaSCs, suggesting that alteration of MaSC frequency and function may lead to transformation and tumorigenesis. Unpublished results indicate that low dose BPA exposure during puberty can alter the number of different lineage MaSCs. Recent animal studies showed that BPA also promoted tumor cell growth through estrogenic signaling implicating the risk of development and progression of mammary cancer by BPA exposure at various time points throughout the lifespan. Thus it is hypothesized that mammary gland exposed to BPA at a susceptible window may lead to its susceptibility to tumorigenesis through a MaSC and/or stem cell niche mediated mechanism. In addition to mice, the intent is to also use common marmoset to determine the effect of BPA on its MaSC function and transformation because nonhuman primates, with their close phylogenetic relationship to humans, can better simulate the effects of physiological and pathological factors on humans. This hypothesis will be tested with three specific aims. First, the effect of BPA will be determined on MaSC function of non-primate and primate subjects. A novel in vitro assay developed in this laboratory will be exploited, instead of cell surface markers that are not specific for MaSCs, to quantify the alteration of basal and luminal MaSC number after systemic BPA treatment in various developmental windows of the mammary gland of mice and marmoset. Second, a determination of the effect of low dose irradiation on the transformation potential of MaSCs derived from BPA-exposed mice will be accomplished. The hypothesis to be tested for this specific aim is that the transforming activity of BPA may be more evident in combination with a DNA damaging agent such as ionizing radiation, which is a known risk factor for breast cancer. Third, the effect of BPA treatment on MaSC function of obese mice and marmosets will be examined. This proposed research will not only answer the question of whether BPA- induced morphogenesis changes in different developmental windows of mammary gland has a stem cell origin, but also shed light on MaSC susceptibility to multiple risk factor-induced cell transformation such as xenoestrogens, irradiation, and obesity, which will have important implications in disease prevention for human breast cancer.